The present invention relates to a comparator and more particularly, to a comparator with adjustable hysteresis using a current mirror circuit.
As shown in FIG. 1, a conventional comparator with hysteresis using resistance ratio comprises a differential amplifier including a pair of transistors Q1 and Q2, the emitters of which are connected to each other and also connected to a resistor R2. An input terminal VIN and a reference voltage terminal (node A), whose reference voltage value is determined by the ratio of resistance values R3-R5, are connected to the differential pair of transistors Q1 and Q2. A transistor Q3 is connected to the common node between the resistors R4 and R5 through a resistor R6, and a transistor Q5 the base of which is connected to the collector of the transistor Q1, is connected between the base of the transistor Q3 and the power supply Vcc. Also, a transistor Q4, the base of which is connected to the base of the transistor Q3, is connected between load resistor R11 and the ground terminal (GND).
When the voltage of the output terminal VOUT of the comparator is low, that is, the voltage of the input terminal VIN is higher than the reference voltage at the node A, the reference voltage is defined as VthL. On the other hand, when the voltage of the output terminal VOUT of the comparator is high, that is, the voltage of the input terminal VIN is lower than the reference voltage, the reference voltage is defined as VthH. The hysteresis voltage V.sub.hys is VthH-VthL.
The hysteresis voltage VY, ca be calculated as follows: ##EQU1##
As described above, the conventional comparator with hysteresis shown in FIG. 1 achieves hysteresis using the resistance ratio, thereby complicating the calculation of the hysteresis voltage. That is, in order to satisfy Eq. (1) , when the transistor Q3 is turned on, the collector-to-emitter voltage of the transistor Q3 must be zero. Accordingly, the hysteresis voltage has some error depending on the saturation voltage Vsat of the transistor Q3. Also, in order to increase the hysteresis voltage, the value of the resistor R6 has to be reduced. But, the reduction of the value of the resistor R6 increases the collector current of the transistor Q3, resulting in an increase of the saturation voltage Vsat of the transistor Q3. Thus, the error in the hysteresis voltage is increased.
Based upon the above, it can be appreciated that the conventional comparator with hysteresis has some disadvantages in that the accuracy of the hysteresis voltage is limited and the adjustment of the hysteresis voltage is difficult. Thus, it is evident that there presently exists a need for a comparator with hysteresis which has a hysteresis voltage which is both accurate and easily adjustable.
The present invention fulfills this need.